Charging 18650 4s setup for battery

Hi all, first post here- just joined for some advice!
Im making a batttery pack using 4 lithium ion 18650 cells in series totalling a charged voltage of about 16.8V
Im aware with the dangers of over charge and discharge etc. I have bought a 4s BMS to control the batteries charge and discharge.

With the batteries being about 16.8 v charged and 14v discharged what voltage do I need to charge them? Do I use a 16/17V charger so that they charge up to the full charged voltage and they 'level' out and thats that or do I use something with a volt or two extra such as an 18-19V charger?

Thanks

Li-Ion should be charged from constant current/ constant voltage sources.

So your constant voltage should be 16.8V - this is VERY important - no higher.

When you initially connect the batteries this voltage will be pulled down, and the power supply will operate in constant current mode.

As the battery charges, the voltage will rise, once it reaches 16.8V the current will start to drop, once the current has dropped to a suitable level (0.05C is a fairly common level) you then stop charging.

Nigel Goodwin said:

Li-Ion should be charged from constant current/ constant voltage sources.

So your constant voltage should be 16.8V - this is VERY important - no higher.

When you initially connect the batteries this voltage will be pulled down, and the power supply will operate in constant current mode.

As the battery charges, the voltage will rise, once it reaches 16.8V the current will start to drop, once the current has dropped to a suitable level (0.05C is a fairly common level) you then stop charging.

Click to expand...

Sound mate. spot on cheers.

On another thread there was a Chinese BMS that limited the current to 65A! It had few other spec's and did not say the required input voltage for the output to be at the charged battery voltage.

Note there are two parts to a lithium "BMS" and lot of the ebay ones only have one part or the other.

You need both undervoltage / overvoltage protection (the common part) AND cell charge balance, the rarer bit.

Without the balance part, the charge will be stopped when any one cell reaches full charge voltage. You can end up with apparent reduced capacity or a non-functional battery pack.

If your module has both parts, fine; if not add the other part!
(Or you can use a balance charger, that has a multi-way auxiliary cable to all the cell junctions in a battery pack).